Recently, a hard disk device is incorporated in a personal computer, a notebook personal computer, and a DVD (Digital Versatile Disc) recording apparatus in order to record data. Particularly, in the hard disk device used in an apparatus such as the notebook personal computer based on portability, a magnetic disk in which a magnetic layer is provided on a glass substrate is used, and magnetic recording information is recorded in or read from a magnetic layer using a magnetic head (DFH (Dynamic Flying Height) head) that is slightly floated on a surface of the magnetic disk surface. A glass substrate is suitably used as the substrate for the magnetic disk because a glass substrate is hardly plastically deformed compared with a metallic substrate.
The magnetic recording density is being increased in order to respond to a demand for an increase of a storage capacity in the hard disk device. For example, a magnetic recording information area is finely formed using a perpendicular magnetic recording system in which a magnetization direction of the magnetic layer is oriented toward a direction perpendicular to the substrate surface, which allows the storage capacity to be increased in a single disk substrate. In order to respond to the further increase of the storage capacity, a floating distance of the magnetic head from the magnetic recording surface is extremely shortened to form the fine magnetic recording information area. With such substrate of the magnetic disk, the magnetic layer is formed flat such that the magnetization direction of the magnetic layer is oriented toward the direction substantially perpendicular to the substrate surface. Therefore, the glass substrate is formed such that surface irregularity of the glass substrate is decreased as much as possible.
The shortened floating distance of the magnetic head may easily cause a head crush trouble or a thermal asperity trouble. Because these troubles are caused by the micro irregularity or a particle on the magnetic disk surface, the glass substrate is formed such that the surface irregularity of an end face is also decreased as much as possible in addition to that of the principal surface.
For example, the glass substrate used in the magnetic disk is manufactured by the following method: specifically, a glass gob made of molten glass (a lump of the glass material) is supplied onto a lower die that is a backing gob forming die; press forming is performed to the glass gob to prepare a sheet glass material using the lower die and an upper die that is a counter gob forming die; and the sheet glass material is formed into a glass substrate for information recording medium (for example, see Japanese Patent No. 3709033).
With the method disclosed in the Japanese Patent No. 3709033, after the glass gob made of the molten glass is supplied onto the lower die, the following steps are performed: a lower surface of a body for upper die and an upper surface of a body for lower die are abutted on together; a thin sheet glass forming space is formed outside a sliding surface between the upper die and the body for upper die and a sliding surface between the lower die and the body for lower die; the upper die is moved down to perform the press forming; and the upper die is moved up immediately after the press forming. Therefore, the sheet glass material that becomes a base of the glass substrate for magnetic disk is formed. Then, the glass substrate for magnetic disk is obtained after a grinding process and a polishing process.
Generally, when forming a gob of molten glass by cutting molten glass with a cutting blade, a portion of the molten glass that contacts the cutting blade is rapidly cooled, thereby forming a cut mark thereon. When press forming a gob including a cut mark, a sheer mark that originates from the cut mark is formed on a sheet glass material. The sheer mark may be observed as aggregate of small air bubbles or dents on the surface.
It should be note that, since two times cutting is required to form a single gob, there normally exists two cutting marks on a single gob, thereby forming two sheer marks accordingly. The cutting mark which is formed earlier may be dissolved again. The sheer mark that originates from the earlier-formed cut mark may be relatively smaller than the sheer mark that originates from the later-formed cut mark, or may be disappeared.
Remaining the sheer mark on a glass substrate for magnetic disk may not only impede read and write of magnetic data, but fail a hard disk drive device due to the contact between a head and the magnetic disk. Thus, it is required to grind and polish the glass substrate so deeply that the sheer mark is surely removed, thereby reducing efficiency of manufacturing of the glass substrate for magnetic disk. In the conventional press forming with an upper die and a lower die, the sheer mark is formed almost in the center of the sheet glass material after the forming, and amount of grinding or polishing may be inevitably greater.
In view of the above, an object of the present invention is to provide a manufacturing method for efficiently manufacturing a glass substrate for magnetic disk while suppressing defects on the principal surface thereof.